This invention relates to a control of high frequency inverters which are phase modulated to produce a dc isolated carrier containing a desired output on its sidebands. The invention is applicable for both dc to dc as well as dc to ac power conversion.
As discussed in parent application Ser. No. 693,955, prior art conventional electrical power inverter devices employ ferroresonant and pulse-width modulation (PWM) technologies. These technologies are low-frequency based and generally employ bulky electromagnetic devices for operation at 60 Hz. In order to overcome the disadvantages attendant PWM technologies, high-frequency links have become preferred for use in grid-connected, photovoltaic inverter applications.
Such applications are generally discussed in:
R. L. Steigerwald, A. Ferraro, F. G. Turnbull, "Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners," to be published in the IEEE Transactions on Industry Applications.
R. L. Steigerwald and R. E. Tompkins, "A Comparison of High-Frequency Link Schemes for Interfacing a DC Source to a Utility Grid," presented at the IEEE Industry Applications Society Annual Meeting, October 1982.
A. Cocconi, S. Cuk and R. D. Middlebrook, "High-Frequency Isolated 4 KW Photovoltaic Inverter for Utility Interface," PCI/Motor-Con Proceedings, September 1983, pp. 39-59.
W. I. Bower, T. S. Key, B. J. Petterson, "Photovoltaic Power-Conditioning Performance Evaluation, Lessons Learned," presented at the 17th IEEE Photovoltaic Specialist Conference, May 1984.
T. S. Key, "Power Conditioning for Grid-Connected P.V. Systems Less than 250 KW," to be presented at the 19th Intersociety Energy Conversion Engineering Conference, August 1984.
V. T. Ranganathan, P. D. Ziogas, V. R. Stefanovic, "A DC-AC Conversion Technique Using Twin Resonant High Frequency Links," presented at the IEEE Industry Applications Society Annual Meeting, October 1982.
It is particularly advantageous that high frequency link technology be employed because it provides dc isolation and inversion without requiring 60 Hz magnetics. Thus, designs can be smaller and lighter, with a lower cost trend due to advances in the semiconductor art.
These high-frequency link inverters operate to convert dc to ac by means of a high-frequency power carrier (typically 20 kHz or above) containing a complex pattern of sidebands. This signal, when rectified and inverted, produces an ac output having a low-frequency base. The process requires several power conversion stages.
Three conversion stages are normally associated with high-frequency links: high-frequency inversion, rectification and low-frequency inversion. For the first two stages, complex snubber networks are usually required. These networks limit semiconductor stress by shaping load lines within acceptable limits. Newly developed snubbers can also perform this function with very low power loss by first storing transition energy in reactive components, and then releasing it to the sources. Examples of snubber networks known in the prior art are disclosed in:
R. Goldfarb, "A New Non-Dissipative Load-Line Shaping Technique Eliminates Switching Stress in Bridge Converters," Proceedings of Powercon 8, 1981; and by PA1 W. J. Shaughnessy, "Modelling and Design of Non-Dissipative LC Snubber Networks", Proceedings of Powercon 7, 1980.
As a further development, resonant power conversion techniques have been recognized as an extremely efficient means of converting power, and have characteristically reduced stress on power semiconductors by switching at substantially zero current or voltages.
For example, such techniques are discussed by I. J. Pitel, the inventor in this application, in U.S. Pat. No. 4,075,476; by F. C. Schwartz and J. B. Klaasens "A 95 - Percent Efficient 1-KW DC Converter with an Internal Frequency of 50 kHz", IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI - 25, No. 4, November 1978, pp. 326-333; and by R. L. Steigerwald, "High-Frequency Resonant Transistor DC-DC Converters" IEEE Transactions on Industrial Electronics Vol. IE-31, No. 2, May 1984.
Two major limitations and reasons why resonant power conversion techniques have not gained increased usage are that they are not easily controlled and work well only at full power output.
With conventional 60 Hz, low frequency inverters, the power conversion circuitry is arranged to circulate periodic reactive power between the source and load to maintain a zero average real power output. This is usually done with non-controlled switches, feedback diodes, or sometimes with 60 Hz reactive components as a storage medium. In general, these inverters are characterized as being unidirectional because real power can only flow from the source to the load.
Conventional high frequency links have complicated the flow of 60 Hz reactive power. Periodic circulation of reactive power to the high frequency link occurs over many cycles, and simple feedback diodes have not been viable. A possible technique would be to double the number of switches in an opposing manner to allow bidirectional power flow. Although this method could probably work, it would increase the cost and reduce the practicality of the device.
In a further development as discussed in parent application Ser. No. 693,955, it is known to provide an electrical power inverter and apparatus which includes a high frequency link for converting DC power into AC power. This apparatus includes a first high frequency module for producing a first AC voltage at a first output frequency, and a second high frequency inverter module for producing a second AC voltage at a second output frequency substantially the same as the first output frequency. The second AC voltage is out of phase with the first AC voltage by a selected angular phase displacement. Mixing means mix the first and second AC output voltages to produce a high frequency carrier which has a selected base frequency impressed on the sidebands thereof. Rectifying means then rectify the carrier, which is filtered by filtering means. Output inverting means invert the filtered carrier to produce AC line voltage and power at the selected base frequency. Phase modulating means then adjusts the relative angular phase displacement between the output voltages of the first and second high frequency modules to control the base frequency of the AC line voltage.
In a particular aspect of the prior system, the phase modulating means includes a line voltage referencing means. An amplitude adjustment means selectively controls the magnitude of the line voltage reference, and a first output comparator means compares the inverter AC line voltage with the line voltage reference.
In the system of U.S. Ser. No. 693,955, in one embodiment, resonant-type inverter moqules are employed to operate near resonance throughout the regulation range to provide a high efficiency of up to about 85%-95%. The highly efficient operation reduces heat generation, saves energy, increases the reliability of the electrical components and reduces the size and number of heat sinks required.
As applied to the system of U.S. Ser. No. 693,955, operation below or above resonance in the prior art has typically involved controlling the driving frequency with respect to resonant frequency. The operation below resonance causes the switches of the system to experience a leading power factor which improves turn-off switching trajectories and allows anode commutation. The operation above resonance improves turn-on switching trajectories and permits use of standard recovery diodes intrinsic to Darlington and FET transistors. In either case it is desirable to operate as close to resonance as possible because it increases output.
Operation at resonance on the other hand is normally in non-controlled applications because the driving frequency is locked to the resonant frequency of the tank circuit. Accordingly, prior to now control of the operation has not been possible.